I have worked with Nikon, Canon and Phase all with long exposures, some totaling up to 55mm.
Nikon and Canon on their higher end cameras offer a dark frame, both will allow you to turn it off. When you stack, it's imperative to turn it off as the resulting dark frame will create a gap. Gaps can be closed in software, but it's an added step that sometimes has trouble depending on the camera lens combination.
It's always been my understanding that the dark frame is more to reduce "stuck Pixels" rather than traditional noise. Stuck pixels will increase over time of exposure and can ruin a frame. Some cameras will show them red, blue other pure white. Either way, they have to be removed before you can run a stacking solution or just from a single long shot. Your DSLR's are all CMOS and thus tend to be a bit less noisy on longer exposures. You still have to watch for heat (both in camera from longer run times and ambient outdoor temps and humidity).
When shooting Canon or Nikon I really don't notice any noise difference with Long Exposure noise reduction turned on. On a short stack say 2 min to 4 min, I will start to see a few stuck pixels over the course of a nights shoot, but nowhere near as many as when I do a single long exposure. It's very easy to test this by taking two single long exposures, say 20 minutes, one with Dark Frame on, one without. The image without LEN on will more than likely will show considerable stuck pixels, the other should be clean. In the old days, before this was available, you always had to shoot one shot with the lens cap on of the same length of exposure then reduce the stuck pixels manually. Never worked as well for me. Also the longer a camera runs, 1 hour, 2 hours, etc. the more stuck pixels tend to show up and most times they are not in the same place each time, thus the need for the internal software (in camera) to remove them.
Since the 5D MKII, Canon has allowed you to run the long noise in the background i.e. buffer. So you could take usually 3 30 to 40 min long exposures before the buffer locked you out. Nikon works just like Phase One (except you can't turn phase off), shoot 1 frame and you are locked out of the camera until the dark frame is written. Before I shot all stacks I preferred Canon over Nikon for this simple reason, i.e. you could get a lot more done in one night before the camera buffered out.
Some of the raw converters can remove some of the stuck pixels, like LR, but I have only found this to work if the image capture is raw. With a jpg, then they are truly stuck. Since most people seem to still shoot jpgs I think this one of the reasons the large camera companies added this feature.
Phase one, with at least the P45+ seemed to do a bit more than just remove the stuck pixels and the dark frame that camera ran IMO was more critical to the end result. Doug or others closer to design by Phase would know more about this. This is CCD and the Dark frame may be more critical to the end result.